A commonly used strategy to detect the presence of pathogens, in biological fluids, is the detection of an antigen (direct method) or a respective antibody (indirect method). However, this strategy, performed with immunometric techniques such as ELISA, IFA or Western Blotting, is limited because of the scarce quantitation accuracy, precision and sensitivity, of the different antibody cross-reactivity and of the impossibility to obtain precocious diagnosis.
Another approach relies on the detection of nucleic acids specific for each kind of molecular target from any biological source, using the amplification by polymerase chain reaction (PCR). This technique, in its more sophisticated version i.e. the quantitative competitive PCR (qcPCR), makes it possible to reach a high sensitivity and a quite accurate quantitative measure, as well as to obtain a diagnosis a short time after contact between the patient and the pathogen. Nevertheless the precision and accuracy of this system is assured in a narrow quantitation range, thus forcing the operator to multiply the number of replies (typically 8) of the sample under investigation; furthermore a long time and additional costs for the amplified product detection steps are necessary.
The first systems that assessed PCR kinetics in real time were based on an intercalating substance such as ethidium bromide. This substance binds to the polymerizing double strand DNA proportionally, enhancing its fluorescence in response to UV excitation; the fluorescence emitted from the intercalated ethidium molecules was registered by a CCD camera in a thermal cycler equipped to irradiate the samples with UV rays and plotted against the amplification cycle number (Higuchi et al., Biotechnology 10:413-417). The main limitation of this technique is that the signal is generated also from the unspecific PCR products.
Subsequently the method known as TaqMan, described in U.S. Pat. No. 5,210,015 was introduced. This method is based on the real time detection of the fluorescence deriving from the degradation, directly dependent on the nascent PCR product, of a labeled probe specifically hybridizing to the segment to amplify, by means of the Taq polymerase enzyme. The PCR reaction mix contains a non-extendable oligonlucleotide probe, labeled with two fluorescent molecules, a reporter at the 5′ end, and a quencher at the 3′ end; the probe sequence must be complementary to a region of the DNA under investigation located between the two annealing sites of the oligonucleotide primers.
During the PCR amplification reaction, the Taq Polymerase enzyme specifically activated by the primers starts duplicating the DNA under investigation; when the enzyme contacts the probe annealed to the DNA, cuts it by its 5′ nuclease activity, removing it and consequently separating the fluorescent molecules; the emission from the reporter fluorochrome becomes thus measurable and, each DNA molecule being accompanied by a reporter molecule release, the total fluorescence is at any time proportional to the amplified DNA amount. The Sequence Detection System 7700 ABI PRISM (produced and distributed by Perkin Elmer) can work both as a DNA amplifier and a collector of fluorescence signals from samples during the PCR reaction. These signals are then processed by a software capable of extrapolating the starting DNA amount in the analyzed samples by a standard curve built with the fluorescence signals from samples with known DNA content. It must be noted that such a system is endowed with two specificity levels: the specific annealing of the primers and the specific annealing of the probe.